The correct option is C Germany's attack on Poland
The expansion policy of Hitler was responsible for the second world war. After first world war Germany had to sign a treaty of Versailles. Due to this treaty, Germany had lost its economic and political power. When Hitler became Chancellor of Germany, he decided to take revenge from Britain and France and wanted to expand the power of Germany. Along with Germany, Italy and Japan also wanted to be a powerful country in the world so these three countries came together and join a group known as Rome, Berlin and Tokyo axis in 1936. Though in 1938 Hitler had seized Czechoslovakia but when Hitler invaded Poland then Britain and France came to protect Poland. In this way along with Germany France and Britain involved in this war and that is how later or sooner other countries also took part in this war and then the second world war started.